Inductive radiation detector

ABSTRACT

A radiation detector includes a block of a material capable of interacting with ionizing radiation to produce charge carriers, an inductor positioned adjacent to the block and having an inductance that depends on a number of the charge carriers in the block, and a sensing circuit coupled to sense a change in the inductance and detect the ionizing radiation base on the change. The sensing circuit may particularly contain an RF synthesizer that drives the inductance, e.g., an LC circuit containing the inductance, and an analyzer that detects changes in the response of the inductance.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent document is a divisional of U.S. patent application Ser. No.15/413,912, filed Jan. 24, 2017, which claims benefit of the earlierfiling date of U.S. provisional Pat. App. No. 62/309,723, filed Mar. 17,2016.

BACKGROUND

Radiation detectors have been used in security, defense, safety,scientific, medical and industrial applications and may detect manytypes of ionizing radiation, e.g., gamma rays, X-rays, and nuclearparticles. One particularly critical use for radiation detectors is forsecurity, for example, cargo inspection to guard against terroristsmuggling a dirty bomb or other radioactive material in commercialshipments, but radiation detectors are not limited to use for cargoinspection or even security applications. More generally, individualradiation detectors and networks of radiation detectors may be widelyneeded, for example, for securing national borders, as part of theregular inventory of law enforcement agencies and fire departments, andfor installations or use at transportation hubs, entertainment venues,road intersections, or any populated areas, for detection of dangerousor illicit radioactive material.

Radiation detectors in many applications may not only need to detectradioactive materials but may also need to identify or differentiatematerials, for example, to avoid triggering false alarms. In particular,a radiation detector may need to distinguish illicit or dangerousradioactive materials from naturally-occurring radioactive material ormedical isotopes. Isotope identification through emission spectrumanalysis is possible but may require expensive gamma-ray detectors withhigh energy resolution and a wide detection bandwidth covering both lowand high energy gamma-rays. Neutron detectors, which are important inscientific research and nuclear reactor operation, may also distinguishillicit radioactive material because few naturally occurring materialsemit neutrons. The most common type of neutron detectors currently usespressurized tubes containing helium-3 gas and are typically bulky,costly, and difficult to configure.

Satisfying the growing need for radiation detectors may requirelow-cost, room-temperature operation, high energy resolution, andhigh-efficiency detection of ionizing radiation including gamma-rays andnuclear particles.

SUMMARY

In accordance with an aspect of the invention, an ionizing radiationsensor may include a semiconductor or insulator block having anelectrical characteristic that measurably changes when the blockinteracts with radiation. A sensing circuit can then monitor theelectrical characteristic and detect radiation when the radiationinteracts with the sensing block. In one configuration, a semiconductoror insulating block sensitive to the radiation is additionally exposedto the radio frequency (RF) electromagnetic radiation, which is affectedby the charge carriers that the radiation frees in the block, and thesensing circuit can detect the radiation based on a change in the RFradiation, e.g., by measuring the RF radiation remaining afterabsorption by the freed charge carriers in the sensing block. Inparticular, RF signal processing may be used to detect the interactionsof radiation with a sensing block, for example, to detect the number ofcharge carriers that are freed by a single quantum of ionizing radiationor nuclear particle and thereby to determine the energy of the quantumof radiation or particle.

One specific implementation is a radiation detector including a sensingblock, an inductor, and a sensing circuit. The sensing block may be madeof a material capable of interacting with radiation to produce chargecarriers in the block and may particularly be a block of semiconductoror insulator. The inductor may be positioned adjacent to or mayincorporate the block, so that a property, e.g., the inductance or thequality factor, of the inductor or a circuit including the inductordepends on a number of the charge carriers in the block. The sensingcircuit consisting, for example, of the inductor and a capacitor formingan LC resonance circuit can then be coupled to the sensing circuit tosense a change in the inductor and thereby detect the ionizing radiationbased on the change.

Another specific implementation is a radiation detector including an RFsynthesizer branch, a sensor branch, and an analyzer branch. The RFsynthesizer branch may generate an input RF signal that the sensorbranch receives. The sensor branch may include a sensing block and acircuit that is associated with the sensing block, and the circuit maybe connected so that the sensor branch has an electrical characteristic,e.g., a resonance characteristic, that depends on charge carrierscreated in the sensing block by radiation. The analyzer branch may inturn be coupled to receive an output RF signal altered by the effect ofthe sensor branch on the input RF signal. (A reference or balancingbranch may be employed to enhance the ability of the analyzer branch tomeasure the changing characteristic of the sensor branch.)

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a radiation detector in accordance with an implementationemploying an inductive coil surrounding a core of semiconductor orinsulator material.

FIG. 2 shows a birdcage coil that may be employed as an inductor in theradiation detector of FIG. 1.

FIG. 3 shows a radiation detector in accordance with an implementationemploying a spiral coil adjacent to a flat surface of a sensing block.

FIG. 4 shows a radiation sensor in accordance with an implementationusing a semiconductor or insulator block and multiple sensing coilsforming LC resonance circuits for radiation detection, imaging, ordirectional detection.

FIG. 5 is a block diagram of a radiation detector in accordance with animplementation employing RF signal processing.

FIG. 6 is a block diagram showing one implementation of a referencebranch suitable for use in the radiation detector of FIG. 5.

FIG. 7 is a block diagram showing one implementation of an analyzerbranch suitable for use in the radiation detector of FIG. 5 and showingtypical electrical signal outputs at key stages of the electronics.

FIG. 8 is a block diagram of a radiation detector and typical electricalsignal outputs at key stages of electronics in accordance with anotherembodiment of the invention.

FIG. 9 is a flow diagram of a process for calibrating a radiationdetector in accordance with an embodiment of the invention.

The drawings illustrate examples for the purpose of explanation and arenot of the invention itself. Use of the same reference symbols indifferent figures indicates similar or identical items.

DETAILED DESCRIPTION

An ionizing radiation sensor may have one or more electricalcharacteristics that depend on the number of free charge carriers in asensing block. The sensing block may particularly contain asemiconductor or insulator having an electrical characteristic thatmeasurably changes when interactions with radiation create chargecarriers in the sensing block. A sensing circuit can monitor theelectrical characteristic of the sensor and detect ionizing radiationwhen an interaction of the radiation with the sensing block changes themonitored electrical characteristic. In one implementation, the ionizingradiation sensor containing a semiconductor or insulating sensing blockfurther includes circuitry having a resonance, e.g., an LC resonance,that changes when ionizing radiation frees charge carriers in thesensing block, and the sensing circuit can detect ionizing radiationbased on a change in the resonance, e.g., changes in the resonancefrequency or quality factor. In particular, radio frequency (RF) signalprocessing may be used to detect the interactions of radiation with asensing block and to detect changes due to interaction of a singlequantum of radiation with the sensing block.

FIG. 1 shows an exemplary implementation of a radiation detector 100including a semiconductor or insulator block 110 coupled to an inductor120 and capacitor 121. Block 110 may, for example, be a solidcylindrical block of an insulating or semiconductor material, andinductor 120 may be a coil of wire wrapped helically in one or morelayer around cylindrical block 110. Capacitor 121 may be a separatecomponent or may result from the inherent capacitance of shielding orconductors as described further below.

The material used in block 110 may be monocrystalline ornon-monocrystalline, for example, polycrystalline blocks or films,chosen according to the radiation to be sensed but is preferablyelectrically resistive, e.g., an insulator or a semiconductor, with lowdensity of charge carriers. For sensing of thermal neutrons, forexample, block 110 may contain ¹⁰B (boron with atomic weight 10), whichis an isotope capable of capturing neutrons and then quickly decaying toproduce ⁷Li (lithium with atomic weight 7) and α (alpha particles).Daughter particles (⁷Li and α) created from the nuclear reactionsgenerally have large kinetic energies and interact with block 110producing free electrons and holes. One suitable boron containingsemiconducting material is hexagonal boron nitride (h-BN), but otherboron containing materials may be used. For X-ray or gamma-raydetection, block 110 may be made of Cadmium Zinc Telluride (CdZnTe,CZT), Cadmium Manganese Telluride (CdMnTe, CMT), Cadmium Telluride(CdTe), Mercury Iodide (HgI₂), Thallium Bromide (TlBr) or other high-Zsemiconductors. An x-ray or gamma-ray interacting with an insulator orsemiconductor may produce one or more electron-hole pairs. In general,when block 110 is exposed to radiation such as x-rays, gamma-rays, ornuclear particles, the reaction of the radiation with the material ofblock 110 produces charge carriers in electrically resistive block 110.The electrical characteristics, e.g., the resistivity and the electricalpermittivity, of block 110 and associated sensing circuitry may changeas a result of charge carrier creation.

Inductor 120, being inductively coupled to semiconductor or insulatorblock 110, may have properties that depend on electrical characteristicsof block 110 and therefore on the number of the charge carriers producedby the ionizing radiation in block 110. Inductor 120, in someimplementations, may be part of an LC circuit that is tuned to aresonant frequency in an RF range or more specifically in an ultrahighfrequency (UHF) range. In one embodiment, inductor 120 may be solenoidcoil wound around semiconductor or insulator block 110 and may beattached to semiconductor or insulator block 110 with an insulatingepoxy. Such a configuration is particularly useful for high-energyapplications such as gamma-rays requiring a large volume forsemiconductor or insulator block 110.

An all-around, light-tight electrical shield 130 may surround block 110and inductor 120 to shield inductor 120 and capacitor 121 from externalelectromagnetic RF interference and to form a cavity that may increasethe uniformity of RF radiation within shield 120 due to reflection fromthe conductive shield walls. The electrical shield may contribute tocapacitance C of capacitor 121 in an LC circuit. In one implementation,shield 130 is cylindrical and made of a conductor such as aluminum orcopper. Shield 130 may also be made of a material having a thickness anda composition that makes shield 130 transparent or nearly transparent tothe radiation to be detected. Leads 122 and 124 from inductor 120 passthrough shield 130 and are electrically connected to a shieldedconductor 140, e.g., to a semi-rigid coaxial cable. In the illustratedconfiguration of FIG. 1, one lead 122 is insulated from shield 130, andthe other lead 124 may be electrically coupled to shield 130 and to theshielding in shielded conductor 140, but alternatively, shield 130 maybe not connected, e.g., may be electrically floating. Shield 130 may begrounded as shown in FIG. 1, but alternatively, shield 130 may be notgrounded, e.g., may be electrically floating. Any space betweensemiconductor or insulator block 110, inductor 120, and shield 130 maybefilled with an insulating epoxy to reduce susceptibility to vibrations.

Inductor 120 with block 110, shield 130, and shielded conductor 140 forma radiation sensor that may act as an LC circuit component, and aresonant frequency of the radiation sensor may be tuned to a frequencyof electromagnetic radiation, related to hardware limitations of thesensor 100, sensing circuit 150 and to a range corresponding to a lowestexternal interference. In any case, a sensing circuit 150 may besurrounded by a conductive shield and through the shielded conductor 140can detect changes in the electrical characteristics of the sensorincluding block 110, inductor 120, and capacitance 121 when ionizingradiation interacts with block 110. Some examples for implementations ofsensing circuit 150 are described in more detail below.

In a case where semiconductor or insulator block 110 has a large volume,a simple wound solenoid coil with semiconductor or insulator core maynot match an input impedance, for instance, 50 ohms, desired for sensingcircuit 150. In such case inductor 120 may have a differentconfiguration. For example, inductor 120 may be a “birdcage coil” suchas birdcage coil 220 of FIG. 2. Birdcage coil 220 generally includes twocircular conductive loops 222 and 224, commonly referred to as endrings, connected by an even number of straight conductive elements 226referred to as rungs or legs. The number of rungs may be chosenaccording to the size of coil 220 and typically ranges from about 8 to32 rungs 226. Birdcage coil 220 also contains linking elements 228 suchas capacitors in end rings 222 and 224 and located between conductinglegs 226, and the arrangement and characteristics of linking elements228 may be selected according to the frequency characteristics desiredfor birdcage coil 220 or the circuit containing birdcage coil 220. In ahigh-pass configuration, birdcage coil 220 may be used with pairs ofcapacitors located along the end rings, to provide a configuration thatapproximates a continuous conducting surface. Alternatively, birdcagecoil 220 may employ other configurations such as those known inRF-transmitters and particularly those used in clinical magneticresonance imaging (MRI) devices. Necip Gurler and Yusuf Ziya Ider, “FEMbased Design and Simulation Tool for MRI Birdcage Coils includingEigenfrequency Analysis,” Proceedings of the 2012 COMSOL Conference inMilan, Italy, describes some aspects of birdcage coil design andimplementation.

FIG. 3 illustrates an ionizing radiation detector 300 employing aninsulator or semiconductor block 310 with an alternative coilconfiguration for an inductor 320. In particular, inductor 320 is aspiral coil adjacent to a flat top surface of block 310. For example,inductor 320 may be a surface coil composed of a single winding ormultiple windings attached to a top surface of semiconductor orinsulator block 320. The coil configuration for inductor 320 may beparticularly useful for sensing neutrons or lower energy ionizingradiation such as X-ray that may employ sensing block 310 with theradiation sensing region (monocrystalline or polycrystalline) located atthe surface opposite to the inductive coil 320. Coil 320 and possiblyone or more sides of block 310 may be within a shield 330 of aconductive material, which may be transparent to the radiation beingdetected. Inductor 320 may have leads coupled to shielded conductor 140and to ground as sensor 100 described above. For X-rays, the radiationsensing region of block 310 may be, for example, formed by attaching ordepositing crystalline or polycrystalline CdZnTe or other high-Z layeron a high resistivity or insulating substrate, and for neutrons, thesensing region of block 31 may be fabricated by forming a layer of asuitable semiconductor or insulator, e.g., ¹⁰B enriched hexagonal boronnitride (h-¹⁰BN), on a high resistivity or insulating substrate.

Inductor 320, in some implementations, may be separated fromsemiconductor or insulator block 310 by a conductive shield 360 made,for instance, from aluminum, with a hole (not shown) under inductor 320.For example, conductive shield 360 may have a diameter corresponding tothe diameter of shield 330 and a hole with a diameter corresponding tothe diameter of the inductor 320. The semiconductor or insulator block310 may be larger than diameter of the inductor 320. With shield plate360 between inductor 320 and block 310, shield 330 may not need tosurround block 310 to shield inductor 320 from external electromagneticinterference. Shield 360 may limit the active volume of the block 310sensitive to the radiation, to approximately the volume located underthe hole in shield 360 under inductor 320. This configuration mayincrease uniformity of the RF radiation in the sensing volume improvingenergy resolution of the sensor. Shield 360 in one embodiment may beconnected to a common ground with shield 330, but in another embodiment,shields 360 and/or 330 may not be grounded.

The surface coil configuration of FIG. 3 is well suited for use inradiation detectors using a sensor array. FIG. 4, for example, shows anionizing radiation detector system 400 employing multiple radiationsensors respectively including multiple surface coils 420 overlying asemiconductor or insulator block 410. Each surface coil 420 may be inclose proximity to but separated from a single semiconductor orinsulator substrate 410, and a conductive shield 460, e.g., a thin sheetof aluminum with openings or holes 465 may extend under coils 420. Forexample, each coil 420 may be about 2 mm in diameter, and each opening465 under an associated coil 420 may be about 2 mm in diametercorresponding to diameter of inductors (coil) 420. Each inductor 420 isfurther connected to an associated capacitor 422 and surrounded by anelectromagnetic shield 430 and may connect to a shielded conductor 440that electrically connects the inductor 420 to a sensing circuit 450.

Ionizing radiation detection system 400 by including multiple inductors420 may allow system 400 to provide spatial or directional informationfor the source of detected radiation. In particular, an inductor 420closer to a radioactive interaction may be expected to provide astronger signal, which indicates a location of the radioactiveinteraction within the area of block 410. With an array of inductors,sensing circuit 450 may be able to analyze the respective signalstrengths to provide an image of the distribution of radiation over thearea of block 410. The directional information may be provided byemploying stack of radiation detection system 400 that will allowsensing circuit 450 to determine a direction to the source of radiationand a location of detected radioactive material. Alternatively, sensingcircuit 450 may sense multiple inductors 420 collectively in parallel toincrease detection efficiency.

A sensor array does not need to use a single sensing block such asemployed in system 400 of FIG. 4. For example, one or more of surfacecoils 420 may be adjacent one or more separate sensing blocks. Also, oneor more sensors in an array may be formed using a coil surrounding asemiconductor or insulator core as shown in FIG. 1 or a flat coil on aseparate semiconductor or insulator block as shown in FIG. 3. In allthese cases, such multi-inductor arrays can be used to locate adirection leading to the radiation source or to form an image of theobject emitting ionizing radiation.

An advantage of an array of sensing coils 420 may be improvedsensitivity when a large sensing block 410 is employed. In particular,use of a large semiconductor or insulator with a single coil as shown inFIG. 1 or 3 may reduce sensitivity of the detector to radiation sincethe inductor may be relatively large and distant from a localizedradiation interaction in the sensing block. A coil array in any of thedisclosed configurations with parallel detection of signals from allcoils may increase sensitivity of the detector system due to thereduction of the semiconductor or insulator volume that must be sensedby each inductor.

The ionizing radiation detectors of FIGS. 1, 3, and 4, as describedabove, each have radiation sensors with inductors inductively coupled tosemiconductor or insulator sensing blocks and capacitance connectedbetween the leads of each inductor with coupling to shieldingcontributing to or replacing this capacitance. The sensors may thus forminductance-capacitance (LC) resonance circuits that are sensitive to thepresence of charge carriers in the semiconductor or insulator block. Asensing circuit may subject such a sensor to a radio frequency (RF)signal, preferably ultrahigh frequency (UHF) signal delivered throughthe shielded conductor and may monitor the effect of the sensor on theRF signal in order to detect radioactive interactions that create chargecarriers.

FIG. 5 is a block diagram showing the functional blocks in oneimplementation of a radiation detector 500 including multiple circuitbranches interconnected through an RF bridge 530. A sensor branch 510,for example, may include circuitry and a sensing block that provide aresonance that is in an RF range and that changes when charge carriersare created in the sensing block. Sensor branch 510 may particularlyinclude a semiconductor or insulator block, an inductor inductivelycoupled to the block, and shielded electrical connections, asillustrated in FIG. 1, 3, or 4 described above, to create together withsensing block a LC resonance. Sensor branch 510 may further containadditional circuitry such as a transformer or other impedance matchingcircuitry.

An RF synthesizer branch 520 applies an RF signal to an RF bridge 530that directs the RF signal to sensor branch 510 and to a referencebranch 540. Some signal may also be directed to a Δ port of RF bridge550. Reference branch 540 may be tuned to provide a response that iscalibrated with the response of sensor branch 510 when sensor branch 510is not subjected to ionizing radiation. For example, reference branch540 may be tuned so that at the RF frequency generated by RF synthesizer520, reference branch 540 provides a signal attenuation that matches thesignal attenuation of sensor branch 510 when sensor branch 510 is notsubjected to ionizing radiation. Additionally, the signal reflected fromreference branch 540 may have a phase selected so that the reflectedsignals from sensor branch 510 and reference branch 540 approximatelycancel each other at a Δ port of RF bridge 530 when sensor branch 510 isnot subjected to ionizing radiation. Further reduction of the signal ata Δ port of RF bridge 530 when sensor branch 510 is not subjected toionizing radiation can be accomplished using impedance tuning ormatching circuitry in the sensor branch 510 to match phase of the signalreflected from the sensor branch 510 to that directed from the RFsynthesizer branch 520 to a Δ port of RF bridge 550 and tuning thesignal reflected from reference branch 540 to approximately cancelsignals reaching a Δ port of RF bridge 550 both from sensor branch 510and from synthesizer branch 520. Such matching may be accomplished usingadditional electronic components or by optimizing electronic components.For example, impedance matching circuitry may employ an added oradjustable capacitor, an adjustable transformer, or length adjustablecoaxial cables to name a few options.

FIG. 6 shows one implementation of a reference branch 640 that could beemployed in radiation detector 500 of FIG. 5. Reference branch 640generally functions to balance the RF bridge connection to the sensorbranch. In the implementation of reference branch 640 shown in FIG. 6,the RF signal from the RF bridge passes through an externally controlledvariable attenuator 642 and then to an externally controlled linestretcher 644. Reference branch 640 may end in either an open circuit ora short circuit. If reference branch 640 ends in an open circuit,reference branch 640 reflects the RF signal with no phase shift, and thesignal returning to the RF bridge then has a phase shift equal to twicethe phase shift of line stretcher 644 and variable attenuator 642. Ifreference branch 640 ends in a short circuit, reference branch 640reflects the RF signal with a 180° phase shift, and the signal returningto the RF bridge has a phase shift equal to 180° plus twice the phaseshift of line stretcher 644 and variable attenuator 642. By adjustingthe attenuation of the variable attenuator 642 and the phase shift ofline stretcher 644, the reflected reference signal can be made to cancelthe RF signal at the Δ port. Reference branch 640 may thus be tuned sothat the RF bridge is in the balanced state where a difference signalfrom the Δ port is zero or small and thus can be amplified withoutoverloading an output amplifier.

An alternative implementation of a reference branch may replace variableattenuator 642 and line stretcher 644 of FIG. 6 with passive components(inductors, resistors and capacitors). Passive components may reduceelectrical noise and increase the stability of the reference branch. Thepassive components may, however, need to be periodically calibrated orfine-tuned to adjust phase and magnitude of signal reflected from thereference branch and to maintain RF bridge balance while the sensorbranch is shielded from the external radiation.

Returning to FIG. 5, difference signal Δ output from RF bridge 530 isthe combination of responses from sensor branch 510, synthesizer branch520, and reference branch 540. Difference signal Δ ideally is flat (oramplitude changes of the balanced signal may be below a desiredthreshold) when sensor branch 510 is not subjected to ionizing radiationand reference branch 540 and matching circuitry are properly tuned orcalibrated. When sensor branch 510 is subject to ionizing radiation, thecharge carriers created by a quantum of the ionizing radiation or anuclear particle change the response of sensor branch 510 creatingtemporary change of the RF signal at the difference signal Δ output.

An analyzer branch 550 is connected to receive the difference signal Δoutput from RF bridge 530 and contains circuitry that analyzesdifference signal Δ to detect ionizing radiation. In particular,analyzer branch 550 may detect or analyze pulses in difference signal Δ.A count or frequency of pulses may, for example, indicate a detectedradiation dose or rate. Analysis of the size or area of the pulses mayindicate the energy of the ionizing radiation interacting with sensorbranch 510. For example, a typical approach for analysis of individualpulses may employ a shaping device that will assure a standard decay ofeach pulse so that pulse area (integration over the pulse) determinesnumber of charge carriers produced and hence energy of the ionizingparticle or quantum. If there is no overlap between pulses produced bymultiple quanta, a multichannel analyzer can determine a spectrum forpulse energies, which may allow identification of the isotoperesponsible for the detected radiation. The intensity of radiation maybe determined from the count rate that may be determined using acounter, which is standard in radiation monitoring and not included inthis description.

FIG. 7 shows one implementation of an analyzer branch 750 that could beemployed in radiation detector 500 of FIG. 5. A waveform 710 illustratesa typical analog waveform for difference signal Δ resulting when twoconsecutive quanta of ionizing radiation interact with a sensor branch.Analyzer branch 750 includes a low noise pre-amplifier and amplifier 752that amplifies difference signal 710 and an envelope detector 754 thatmay remove or filter out a carrier RF frequency generated by the RFsynthesizer branch. For example, envelope detector 754 may include arectifier and a low pass filter that produces an amplitude signal havinga typical waveform 755 that may result from the difference signalwaveform 710. The rectifier may be a diode, for instance, a zero biasSchottky diode, and the low-pass filter may be an integrator or anyfilter circuit capable of removing high frequency (carrier) componentfrom the difference signal. Envelope detector 754 could alternatively beimplemented using a variety of analog or digital circuits, and onealternative implementation is described below with reference to FIG. 8.

The properties of the semiconductor or insulator block and theinteraction with ionizing radiation (as well as the implementation ofthe detector electronics) determine the shape of pulses after envelopedetection, e.g., the shape of waveform 755. The interaction or stoppingtime for a single quantum of radiation such as a single alpha particleor an individual gamma-ray photon in semiconductors, insulators, and ingeneral in solids, is typically a few picoseconds. Therefore, the risetime of a pulse is essentially determined by a time constant of the LCcircuit in the sensor branch and the bandwidth of the detectionelectronics, which, for instance, may produce a pulse with a rise timein a range of a few or several nanoseconds. The decay time orpersistence of the charge carriers in the semiconductor or insulatorsensing block is determined by properties of the sensing block, e.g.,the effective recombination lifetime of the electron-hole pairs producedby the quantum of radiation. For presently available semiconductors suchas CdZnTe, the effective recombination lifetime can be a severalmicroseconds. Hence the shape of pulses in the envelope signal mayresemble a step-like function, which is essentially of the same shape asoutput signals exiting charge sensitive preamplifiers often used inconventional radiation detectors. (See, for example, the overview ofpulse processing for conventional semiconductor radiation detectorsdescribed by G. F. Knoll, Radiation Detection and Measurement (4th ed.),John Wiley & Sons, Inc., copyright 2010). Accordingly, shaping deviceand discriminator 756 and multi-channel analyzer 758 may be of the sametypes known for conventional, current sensing radiation detectors.

FIG. 8 shows a more detailed block diagram of an alternativeimplementation of a radiation detector 800. Radiation detector 800includes a sensor branch 810, an RF synthesizer branch 820, and areference branch 840 that are interconnected through an RF bridge 830.Sensor branch 810 includes a block 812 of a low carrier densitymaterial, e.g., semiconductor or insulator, that interacts with ionizingradiation to be detected, an inductor 814 inductively coupled to block812 and having associated circuitry, e.g., with capacitance formingparallel LC circuit with inductor, and a shielded conductor 816, whichmay be similar or identical to systems described above with reference toFIG. 1, 3, or 4.

Sensor branch 810 further includes a transformer 818 to impedance matchinductor-capacitor sensor circuit to hybrid 830 but transformer 818 maynot be required for other LC circuits. Transformer 818 may particularlybe employed when the impedance of sensor branch 810 would otherwisediffer from the impedance desired for inputs of RF bridge 830. In oneembodiment, coaxial cable 816 of sensor branch 810 is connected to180-degree port of hybrid 830 through RF transformer 818, such as twotransformers of model TC4-11, manufactured by Mini-Circuits, Brooklyn,N.Y., connected in series. The use of transformer 818 may ensure thatthe impedance of sensor branch 810 matches the input impedance, e.g., 50Ohms, of hybrid 830. In an alternative embodiment, the diameter ofdetection coil and the number of windings of inductor 814, even for alow-energy radiation application, are selected such that sensor branch810 matches the input impedance of the RF hybrid 830 without usingtransformer 818. Coaxial cable 816 may be connected directly to180-degree port of hybrid 830 without the need for transformer 818 ifthe impedance of sensor branch 810 without transformer 818 matches theinput impedance of RF bridge 830. In a high-energy application using,for instance, the solenoid coil of FIG. 1 or the birdcage coil of FIG.2, the impedance of sensor branch 810 can be made to match the inputimpedances of the hybrid 305, and cable 818 can again be connecteddirectly to the 180-degree port of the hybrid without the need fortransformer 818.

RF synthesizer branch 820 includes a programmable RF frequency source822 that generates an RF signal. RF frequency source 822 may, forexample, be a programmable frequency synthesizer that is controlled byan external controller (not shown). A power splitter 824 receives the RFsignal from RF source 822, sends a first portion of the RF signal to RFbridge 830, and sends a second portion of the RF signal to the analyzerbranch circuitry, e.g., through an amplifier 862 to an envelope detector854. In an exemplary implementation, RF source 822 may be an RFfrequency synthesizer model LMS-152D or LSG-152 available from VaunixTechnology Corporation of Haverhill, Mass. and is capable of generatinga signal in a frequency range from 250 MHz to 1,500 MHz. In anotherexemplary implementation, RF source 822 may be an RF frequencysynthesizer model TRF3765 VCO manufactured by Texas Instruments ofDallas, Tex. which may be boosted by a power amplifier such as a PHA1+manufactured by Mini-Circuits of Brooklyn, N.Y.

RF bridge 830 may be constructed using a 180-degree hybrid coupler,which is a passive RF coupling device having four ports. An example of asuitable hybrid coupler is a model HC-W500-MS available from UniversalMicrowave Components Corporation of Alexandria, Va. RF synthesizerbranch 820 applies an RF signal to the Σ port of the hybrid coupler. RFbridge 830 sends half of the RF signal into sensor branch 810 and theother half of RF signal into reference branch 840. Any power reflectedfrom reference branch 840 appears in phase at a Δ port of RF bridge 830.Any power reflected from sensor branch 840 appears 180° out of phase ata Δ port of RF bridge 830.

The difference signal, which result from combining returning in-phaseand out-of-phase signals, exits RF bridge 830 from the Δ port to theanalyzer branch circuitry of radiation detector 800. In particular, thedifference signal passes through a low-noise amplifier 852, and reachesenvelope detector 854. In the illustrated implementation, envelopedetector 854 includes an RF phase detector 864 that demodulates thedifference signal into two output signals representing quadraturecomponents of the incoming difference signal. Output signals from phasedetector 864 enter an I/Q quadrature demodulator 866. I/Q quadraturedemodulator 866 may be, for instance, model ADL5387 from Analog Devices,Inc., which provides a demodulation bandwidth of about 240 MHz. I/Qquadrature demodulator 866 passes a demodulated signal through low-passfilter 868, which removes high frequency (carrier) component from thesignal leaving an envelope signal. Waveform 855 illustrates a typicalenvelope signal in a case when two consecutive quanta of ionizingradiation interact with block 612 and cause the difference signal tohave the waveform 835.

Radiation detectors such as disclosed herein may be calibrated at thefactory, during start up or use of the detectors, or by a user. FIG. 9is flow diagram of an exemplary calibration process 900, which may beperformed automatically by a controller or manually by a user ormanufacturer. Process 900 is described below with particular referenceto radiation detector 800 of FIG. 8 but is also applicable to otherradiation detector implementations.

Calibration process 900 may require that the sensor branch of theradiation detector, e.g., sensor branch 810 of radiation detector 800,not be exposed to ionizing radiation, and a sub-process 910 shieldssensor branch 810 of a radiation detector 800 by employing an additionalshield on block 812 or by keeping radiation detector 800 in anenvironment known to be relatively radiation free. To achieve maximumsensitivity, process 900 calibrates or sets the frequency of the RFsignal generated at or near the resonance frequency of the input LCcircuit of the sensor branch, e.g., of sensor branch 810. For thefrequency calibration, a process 920 sets the generated RF power at alow level, e.g., RF power source 822 may be set at its minimum powerlevel. Process 920 may also initially set the reference branch, e.g.,variable attenuator 842 in reference branch 840, for maximumattenuation. Setting the RF synthesizer branch to low power output mayprevent system overload, e.g., overload of amplifier 852.

A process 930 is performed to find a coarse matching of the frequency ofthe generated RF signal and the resonance frequency of the sensor branchof the detector. For example, the output frequency of RF synthesizerbranch 820 may be scanned through at least a portion of its frequencyrange while the response of the sensor branch is monitored. Theresonance frequency may be found when the magnitude of the amplifieddifference signal, e.g., output from low-noise pre-amplifier 852measured with an external RF power meter is minimized since the sensorbranch is most sensitive to absorption of RF power at the resonancefrequency. Optionally, this frequency search procedure 930 may berepeated with different output RF power levels until the measured RFpower of the difference signal reaches a predetermined nominal power.The nominal power level may be selected according to the linearity ofthe analyzer branch, particularly amplifier 852 in detector 800 or therange in which detection system is response linearly depends on theinput power. In the illustrated implementation of process 900, aftereach repetition of frequency matching process 930, a decision step 932determines whether RF power levels are acceptable or optimal forradiation detection, and if not, step 934 changes, e.g., increases, thegenerated power before process 900 repeats the coarse frequency matchingprocess 930. In some radiation detector implementations, the adjustmentor matching to the resonance frequency is computer controlled andperformed periodically by automatically scanning the output frequency ofRF signal generator.

An alternative embodiment of frequency matching process 930 may set theRF synthesizer branch at a predetermined RF frequency, and then adjust acharacteristic of the sensor branch. For example, a capacitance of theLC circuit in the sensor branch may be adjusted for the resonancefrequency to match the frequency generated by the RF synthesizer branch.

Once the resonance frequency and the generated RF frequency match andpower levels are adjusted, a calibration process 940 balances the RFbridge. For example, starting from a minimized difference signal whichprovides a coarse null achieved during frequency matching, the phase andmagnitude of the difference signal may be adjusted by adjustingreference branch 840. For example, a coarse adjustment sub-process 942may adjust line stretcher 842 and variable attenuator 844 in referencebranch 840 of FIG. 8 in steps, while the difference signal afteramplifier (or only preamplifier) 852 is measured using an external RFpower meter. The stepping of the reference branch parameters may berepeated until a coarse minimum of the difference signal is achieved. Afine adjustment process 944 to the phase and attenuation may be based onthe magnitude of the envelope signal, i.e., based on measurement of thesignal output from envelope detector 854 with an external meter, sincethe envelope signal may provide higher sensitivity. The fine adjustmentsteps may be repeated until a fine minimum of the envelope signal isachieved. In one embodiment, the coarse and fine minima of signal aredefined as values within predefined multiples of the standard deviationof the noise limits of an external meters used to measure the differenceand envelope signals, respectively. RF bridge balancing process 940allows the signals from the sensor branch and the reference branch to becancelled accurately, preventing saturation of the amplifiers andincreasing sensitivity of the radiation detector to small variations inthe difference signal. In one embodiment, the fine adjustment of themagnitude and phase of reference branch 840 is computer controlled andautomatically performed at periodic intervals.

In another embodiment, in which reference branch 840 relies on passivecomponents such as inductors, resistors, and capacitors, the adjustmentof the phase and magnitude of the RF signal reflected from referencebranch to maintain RF bridge balancing may be performed periodicallywhile the sensor branch is shielded from the external radiation.

After RF bridge 830 is balanced, a change of carrier density insemiconductor or insulator block 812 affecting the impedance of sensorbranch 810 results in a loss of balance of RF bridge 830 and acorresponding change in the difference signal exiting the Δ port. Theresulting RF difference signal is amplified in amplifier 852 and passedto envelope detector 854. The output signals of envelope detector 854,e.g., waveform 855 showing two consecutive pulses, may be submitted toshaping device and discriminator 856, and multi-channel analyzer 858.

Detection of ionizing radiation using a semiconductor or insulatorsensing block may be achieved as described above using an inductivedetection mode. The inductive mode of operation may achieve advantagesover conventional radiation detectors that have used a “photoconductor”(or current measurement) mode of operation. In particular, the inductivemode of operation disclosed herein can achieve high energy resolutionand sensitivity because the charges produced by ionizing radiation areessentially detected at the position that these charges are createdwithin the time much shorter than effective carrier recombinationlifetime. This eliminates a major limitation of the conventional currentmeasuring detectors that can only detect charge carriers createdsomewhere in the block after they are collected at the electrodes. Thephotoconductor mode detectors may thus decrease the energy resolution ofdetectors due to variation of charge collection times associated withrandom distribution of radiation quanta interactions within the volumeof detector and variation of charge collection time due to trapping ofcharges before charges reach electrodes. A still further advantage overcurrent measuring detectors is that the difference in mobility ofelectrons and holes does not affect inductive charge carrier detection,and inductive detection does not suffer variable loss of the signalamplitude that occur in current measuring detectors due to chargecarrier recombination and trapping on their path to the chargecollecting contacts. The larger the detector active volume in currentmeasuring detectors the larger degradation of the energy resolution dueto the position-dependent charge collection and carrier trapping. Athigh radiation energy requiring large active volume, the effect ofvariable charge collection time can be a dominant contributor todegradation of the energy resolution of radiation spectra, especiallythose taken under high rate conditions susceptible to pulse pile-up. Theinductive mode of operation may also reduce the requirements on theresistance of a semiconductor sensing block and on the bulk and surfaceleakages of the block.

Another advantage of the inductive mode of operation is that in thismode of operation charges produced by ionizing radiation are detectedwithin the time much shorter than effective carrier recombinationlifetime (several nanoseconds vs. several microseconds) and the leadingedge and the height of the pulse produced by interaction of a quantum ofradiation with the sensing block is essentially independent of thecarrier recombination and trapping. With decay of the pulse establishedby a shaping device, the area under the pulse after shaping device isalso essentially independent of the carrier recombination and trapping.This allows use of the lower quality sensing materials eithermonocrystalline or non-monocrystalline. Inductive detectors may, forexample, use polycrystalline, or composite materials instead of the highquality monocrystalline materials that the conventional, currentmeasuring, detectors require. Use of lower quality materials andincreased flexibility of sensing block design may substantially lowercost of the radiation detectors.

Although particular implementations have been disclosed, theseimplementations are only examples and should not be taken aslimitations. Various adaptations and combinations of features of theimplementations disclosed are within the scope of the following claims.

What is claimed is:
 1. A radiation detector comprising: a block of amaterial capable of interacting with radiation to produce chargecarriers; an inductor positioned adjacent to the block and having aninductance that depends on a number of the charge carriers in the block;and a circuit coupled to sense a change in the inductance and detect theradiation based on the change.
 2. The radiation detector of claim 1,wherein the material is an insulator or a semiconductor.
 3. Theradiation detector of claim 1, further comprising an electromagneticshield around the inductor and isolating the inductor from externalelectromagnetic RF interference.
 4. The radiation detector of claim 3,wherein the electromagnetic shield forms an RF cavity containing theinductor.
 5. The radiation detector of claim 1, wherein the inductorcomprises a helical coil that surrounds the block.
 6. The radiationdetector of claim 1, wherein the inductor comprises a birdcage coil. 7.The radiation detector of claim 1, wherein the inductor comprises aspiral coil adjacent to a flat surface of the block.
 8. The radiationdetector of claim 1, wherein the radiation is selected from a groupconsisting of X-rays, gamma-rays, or nuclear particles includingneutrons.
 9. The radiation detector of claim 1, wherein the material ofthe block is selected from a group consisting of a semiconductorcontaining ¹⁰B, Cadmium Zinc Telluride (CdZnTe), Cadmium ManganeseTelluride (CdMnTe), Cadmium Telluride (CdTe), Mercury Iodide (HgI₂),Thallium Bromide (TlBr), and other high-Z semiconductors.
 10. Theradiation detector of claim 2, wherein the material of the blockcomprises a mono-crystalline, poly-crystalline or non-monocrystallinematerial.
 11. The radiation detector of claim 1, wherein the circuitcomprises: a sensing circuit; and a capacitance between leads of theinductor whereby an LC circuit that is coupled to the sensing circuit.